Agents window send fails when Git LFS is unavailable

- Copilot Chat Extension Version: Bundled extension from the current VS Code main build
- VS Code Version: 1.129.0
- OS Version: macOS
- Feature (e.g. agent/edit/ask mode): Agents window, first message in a worktree-isolated session
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): GPT-5.6 Sol
- Logs: `Error: (sendFailed) Error: git -c exited with code 128: git-lfs filter-process: git-lfs: command not found`

Steps to Reproduce:

1. Open a repository with required Git LFS filters while `git-lfs` is unavailable from the Agent Host's `PATH`.
2. In the Agents window, create a worktree-isolated session and send its first message.
3. Observe that first-send materialization fails while checking out the agent worktree, so the message is not sent.

Expected: Sending succeeds. When Git LFS is unavailable, checkout leaves LFS pointer files intact.

Actual: Git inherits the repository's required LFS filter and exits with code 128, surfacing the checkout failure as `sendFailed`.